Introduction
Kitsap County saw an opportunity for a grand multi-dimensional improvement project in Manchester, Washington, due to its “aging and undersized outfall”¹ and no existing stormwater treatment in the contributing drainage area. This infrastructure retrofit project would ultimately “treat stormwater from a larger drainage area as well as add some [recreational] amenities”.

According to the Manchester Stormwater Retrofit Study, “Green infrastructure mimics nature to manage stormwater so that it returns to our aquifers, beaches, lakes and streams without flooding or polluting.”¹ This new park will utilize nature and accredited products to serve the long-term benefits of the community and environment. Treating the stormwater to the outfall was essential to the project, because the outfall flowed towards a busy public shoreline.

The Challenge and Solution
The Manchester, Washington Stormwater Retrofit and Community Park was the first of Kitsap County’s stormwater parks. The county challenged civil engineering firms, Parametrix and NW Cascade Construction to create this dynamic infrastructure.

The project was a high-profile retrofit project with two fundamental goals. First, the new park aimed to treat high-capacity stormwater to enhance treatment standards, while creating a new public space for the community. Second, the park needed to be a “green” stormwater solution. Low Impact Development (LID) techniques had to be utilized to meet this requirement and maintain eligibility for Ecology Grant funding. The project incorporated bio retention systems like the spiral rain garden, stormwater biofiltration systems like the Modular Wetlands’ MWS Linear and advanced screening and separation systems like the NSBB. The MWS Linear provides treatment for the downtown Manchester area.

The project had two distinct challenges, but any solution would still have to overcome any and all prevailing retrofit challenges.

The first challenge was treating large water volume with concentrates of sediment from downtown Manchester. It was necessary to find a stormwater solution that would treat high levels of zinc, copper, phosphorous and e-coli bacteria for 3-4 acres of water runoff. This runoff flowed continuously (not just during large amounts of rainfall) and a unit was necessary to treat this type of flow. Furthermore, the unit had to maintain a shallow install depth. The Modular Wetland’s MWS Linear was the solution. The MWS Linear can be installed in tight, shallow locations and uses “WetlandMEDIA”, which contains no organics. These are all standards and features that have allowed the MWS Linear to meet the restricted conditions, high treatment requirements, and maintain grant funding eligibility by the Washington State Department of Ecology – A General Use Level Designation (GULD) for Basic, Enhanced/Metals and Phosphorus treatment.

The second challenge was working with high-capacity water flow and large sediment build up from the surrounding community. The NSBB was commissioned to handily control these high levels of sediment, essential to the functionality of the park; otherwise the downstream vault media and spiral rain gardens would over load and fail. The NSBB also met retrofitting challenges, which required a shallow depth pretreatment system and it’s the only unit that can be installed shallow and still maintain minimum maintenance and high treatment performance levels.

The Result

Retrofitting within a limited space, managing high-capacity stormwater flows and treating specific pollutants were the main concerns of this project. The NSBB’s three-chamber system design allows trash and organic debris to be captured both in dry state and at the same time significantly reducing sediment loading to the downstream spiral rain garden and bio retention systems. The NSBB is easily retrofitted into the preexisting shallow system, where the MWS Linear provided the enhanced treatment for the downtown area. The MWS Linear delivers an 85% TSS removal rating and it was successful in eliminating key pollutants derived from the upstream downtown area: Zinc, copper, phosphorous and e-coli bacteria.

As a community gathering place, it was important to use stormwater treatment solutions that were both functional and pleasing to the eye. The MWS Linear was designed as an open-top planter that was planted with drought tolerant native grasses that intergraded well into the surrounding landscape. The NSBB and MWS Linear were able to conveniently integrate into the retrofit, meet high treatment expectations, low maintenance and landscape appeal for Kitsap County’s first-ever stormwater park.